Tripplehorn will make her debut on the Season 8 premiere, filling the void left by Paget Brewster's Emily Prentiss, who exited in the season finale. And though Messer previously described Alex as a "threat" to the team, she clarifies that it's merely a case of a newbie joining a tightknit group."Anytime there's a very close coworker environment, as we have here, it's hard," Messer tells TVGuide.com. "There's just a natural hesitation to accept, but in that friendly way. She certainly doesn't have an ulterior motive or anything."What else can we expect from Alex? Get the scoop on her and more below.1. Double duty: Alex is also a linguistics professor at Georgetown, where Reid (Matthew Gray Gubler) has guest-lectured, but don't think she's an FBI noob. "She's been in the Bureau since she was 24, so it's all she's ever known," Messer says. "She's never left; she does both. She's like agent by day, professor whenever she can. Hotch [Thomas Gibson] and Rossi [Joe Mantegna] have known her from her career in the Bureau." Also look for a few scenes of Alex teaching in future episodes.2. A history of backstabbing: Reid isn't the only one with whom Alex has a past. She also goes way back with Strauss (Jayne Atkinson), who let Alex take the fall in a case once — and she hasn't gotten over it yet. "There are some professional backstabbing issues with Strauss," Messer says. "Blake talks to Strauss in a way no one else does. She's a woman, they've got a history clearly, and there's no patience for it. It's like, 'I've got a plane to catch, so are we done?' Someone's gonna put Strauss in her place and it's gonna be Alex. She's got a different voice and I think you guys will see it right away. She's very grounded and she's no bullsh--."

3. One big happy family? The season will open with Alex already on the team, and with Morgan (Shemar Moore) and Garcia (Kirsten Vangsness) meeting her for the first time when they return from a temporary assignment in London. And they don't exactly get off on the best foot. "[Alex] and Morgan, clearly do things differently, and they discuss it that episode," Messer says. "Garcia is just like, 'Who is this new person?'" But there won't be any drawn-out intra-team drama. "They work through everything. By the end of that episode, they have mended any fences. Garcia says, 'Can we start over?' We just wanna tell our story with the heroes that we know and trust. They welcome somebody new into their team, and we hope the audience will too."4. Long-distance love, Part 1: Alex has been in a long-distance relationship for a while, and her significant other may eventually pop up. "They have a Skype relationship right now," Messer says. "He's with Doctors Without Borders, so this has always been their life."5. Long-distance love, Part 2: Speaking of long-distance relationships, Hotch will embark on one when Beth (Bellamy Young), who is now a regular on Scandal, returns to explain her future absence. "They're not breaking up," Messer says. "He's not a serial dater, so we didn't want to introduce some new love and have them break up so soon. ... It's gonna be long-distance, so he and Alex have something in common."6. Other matters of the heart: As previously reported, Reid will at long last get a girlfriend, whom he meets over their mutual love of an author. Gubler is "dubious" about Reid dating, but Messer thinks the awkward genius will learn to navigate the troublesome waters. "It's gonna be a very interesting relationship," she says. "It'll be very dramatic and I think fans will be caught off-guard with what we do." Garcia and Kevin (Nicholas Brendon) will also have some ups and downs, but Messer says they'll probably reunite by the end of the season. As for newlyweds JJ (A.J. Cook) and Will (Josh Stewart), home life is going to be kosher after Season 7's life-threatening finale.

7. Season-long unsub: For the first time, the drama will feature a season-long unsub, something producers have wanted to do for a while but had never gotten approval on until now. "The unsub in the premiere sews mouths shut, but by the end of the premiere, he's no more. And a couple of episodes later, the team finds out that back in Texas, where we solved that other case, a victim just showed up with her mouth sewn shut," Messer explains. "So then you find out that this guy is replicating crimes we've solved. We're calling him The Replicator, unofficially. We'll probably see him five or six times [throughout the season]."8. A Prentiss return? Though the show would love to welcome back Brewster any time, Messer doesn't foresee it happening soon. "It would be fantastic, but it's too early to make those decisions. It takes a lot of people to sign off on things like that," she says. But Prentiss is definitely not forgotten in the BAU, as the show will name-drop her whenever it can. "In the premiere, we learn that Morgan and Garcia visited with Prentiss while they were in London," Messer says. "We say, 'Emily sends her love and tea of the month.' Every once in a while, we wanna be like, 'Did you try this one yet?' We'll mention her as often as possible. She's gonna be loved."Season 8 of Criminal Minds premieres Wednesday, Sept. 26 at 9/8c on CBS.